The present invention relates to an air/fuel ratio sensor which is inserted into the exhaust gases of a gas appliance, an oil appliance or an automobile engine to detect the ration between the air and the fuel so as to control it.
Generally, a sensor of the concentration cell type which has a platinum black electrode on both sides of a stabilized or a partially stabilized zirconia electrolytic plate is known. Suppose the oxygen partial pressures on both the electrodes are respectively P'O.sub.2 and P"O.sub.2, the sensor causes such electromotive force E shown in the following equation (1) between both the electrodes. EQU E=RT/4F.multidot.ln(P'O.sub.2 /P"O.sub.2) (1)
wherein R is a gas constant, T is an absolute temperature, and F is a Faraday constant.
Accordingly, when the temperature T and the electromotive force E are measured with one electrode being exposed to the atmosphere, which is approximately constant in oxygen partial pressure as in air, the oxygen partial pressure on the other electrode may be detected.
In order to determine the oxygen partial pressure in the exhaust gas, a cylindrical pipe 101 blocked at one end is composed of zirconia electrolyte as shown in FIG. 1, electrodes 102, 103 made of platinum black connected with an anode terminal 104 and a cathode terminal 105 inside and outside it, and they are inserted into the exhaust pipe from the blockade side, so that the measuring operation is effected with the electrode 102 on the inner side being exposed to the air, the electrode 103 on the outer side being exposed to the exhaust gas. However, in the sensor of this concentration cell type, the electromotive force varies in proportion to the logarithm of the oxygen partial pressure, so that it is suitable when the oxygen partial pressure in the exhaust gas varies through units with the composition as the border as it is in the detection of the equivalent point of the combustion. However, as the electromotive force is small in change when the controlling operation is performed through the detection of an arbitrary air/fuel ratio, a high precision of the S/N ratio can not be realized.
In order to optionally detect the air/fuel ratio, the above portion of the external electrode 103 is covered with porous film 106 made of Al.sub.2 O.sub.3 or MgAl.sub.2 O.sub.4 as shown in FIG. 1, the DC voltage is applied from the terminals 104, 105 between both the electrodes 102, 103 to measure the diffusion limit current. As the diffusion limit current il changes in proportion to the oxygen partial pressure P'O.sub.2 in the exhaust gas as shown in the following equation (2), the arbitrary air/fuel ratio may be detected through the measurement of it. EQU il=4FDAP'O.sub.2 /l (2)
wherein D is a diffusion constant of oxygen gas, A is the total of the sectional area of holes in the porous film 106 covering the external electrode 103, l is film thickness. Among these constants, D changes depending upon temperatures, so that the applied DC voltage by which the diffusion limit current is provided must be required to be increased more, depending upon the temperature, when the temperature is lower as shown in FIG. 2. To avoid such a complication in controlling DC application voltage, a heater 107 which is connected with the terminal 108 has to keep the temperature approximately constant.
Also, the construction shown in FIG. 3 is known as the other conventional diffusion limit current type sensor. In FIG. 3, an oxygen pump cell 111 made of zirconia electrolytic plate has electrodes 112, 113 provided on both faces thereof. A detection cell 114 made of the zirconia electrolytic plate has electrodes 115, 116 on both faces thereof. A heater 118 is buried in the base plate 117. A spacer 119, a spacer 120 are respectively interposed between the oxygen pump cell 111 and the detection cell 114, and between the detection cell 114 and the base plate 117. Diffusion gap 121 is provided between the oxygen pump cell 111 and the detection cell 114, and an air layer 122 is provided between the detection cell 114 and the base plate 117. A gas introduction hole 123 is formed in the oxygen pump cell 111.
In this sensor, the current of the oxygen pump cell 111 is previously settled to a fixed value and direction, and a fuel injection is controlled by a potential of the detection cell 114.
However, in the conventional construction shown in FIG. 1, the temperature of the exhaust gas considerably changes through the operating conditions as in an automobile. It is difficult to keep the sensor temperature constant at high speed. In the sensor, the controlling operation can not be performed in the case of fuel excess.
In the conventional construction shown in FIG. 3, the problems of the conventional embodiment shown in FIG. 1 may be removed. However, the construction may be made complex and larger as the detection cell 114 is provided separately from the pump cell 111. Also, using air electrode as a reference electrode of the detection cell 114 gives a difficult to provide for equipment.